The present invention relates to fluid coupling devices, and more particularly, to such devices which are capable of changing between the engaged and disengaged conditions, in response to variations in a predetermined temperature condition.
A fluid coupling device of the type to which the present invention relates typically includes an input coupling member and an output coupling member. The output coupling member cooperates with a cover assembly to define a fluid chamber and a valve plate separates the chamber into a reservoir chamber and an operating chamber. The input coupling member is rotatably disposed in the operating chamber, and cooperates with the output coupling member to define a viscous shear space, such that torque may be transmitted from the input member to the output member by means of a viscous shear fluid.
The valve plate includes a valving arrangement operable in response to variations in ambient air temperature to permit fluid to flow from the reservoir, through a fill opening defined by the valve plate, into the operating chamber. Typically, such fluid couplings include a discharge port defined by the valve plate and disposed near the outer periphery of the operating chamber, with some form of pumping element, such that a small quantity of fluid is pumped from the operating chamber back to the reservoir chamber during normal operation.
With the advent of smaller automobile engines, operating at relatively higher speeds and temperatures, it becomes increasingly common for the temperature of the fluid in the coupling device to exceed a predetermined maximum temperature. When this occurs, the silicone fluid typically used in such coupling devices undergoes a process in which the fluid first "droops", i.e., there is a physical breakdown of the polymer chains such that the fluid viscosity decreases and the torque transmitting capabilities of the device decreases substantially. Then, with continued excess fluid temperature, the fluid again begins to cross-link, but does so excessively, and eventually "gels" or becomes almost solid. When the fluid reaches this condition, the coupling device operates as if it were solid, with no slip speed in the engaged mode, and no capability of operating in a disengaged mode.
Those skilled in the art have attempted to overcome the problems described above by various means. U.S. Pat. No. 4,662,495 discloses a fluid coupling device having an IBM (internal bimetal) which responds to internal fluid temperature, and acts independently of the normal, external bimetal which senses ambient air temperature. As the internal fluid temperature exceeds a predetermined maximum, the IBM is configured to overcome the normal springiness of the axial valve arm, and move the valve arm from an open position (displaced away from the valve plate and the fill port) back toward the valve plate, covering the fill port. Although the device of the invention would theoretically operate satisfactorily, the provision of an additional bimetal element, within the fluid coupling device, adds substantially to the manufacturing cost and complexity of the device, as well as adding cost and complexity to the assembly and testing procedures required. Furthermore, although the addition of an IBM is suitable for use with an axially movable valve arm, an IBM is not especially suited for use with a rotary valve arm of the type which is fixed relative to the inner end of a bimetal coil, by means of a valve shaft.